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Published on November 13, 2008

Author: sat_innova

Source: authorstream.com

Slide 1: Tolerancing: Control of Variability Objectives : Objectives Understand description and control of variability through tolerancing Understand various classes of fits Introduce multiple part tolerancing Tolerance : Tolerance Tolerance is the total amount a dimension may vary. It is the difference between the maximum and minimum limits. Ways to Express: Direct limits or as tolerance limits applied to a dimension Geometric tolerances A general tolerance note in title block Notes referring to specific conditions 1. Direct Limits and Tolerance Values : 1. Direct Limits and Tolerance Values 1. Direct Limits and Tolerance Values – Plus and Minus Dimensions : 1. Direct Limits and Tolerance Values – Plus and Minus Dimensions 2. Geometric Tolerance System : 2. Geometric Tolerance System Geometric Dimensioning and Tolerancing (GD&T) is a method of defining parts based on how they function, using standard ANSI symbols. Feature Control Frame Concentricity Symbol 3. Tolerance Specifications in Title Block : 3. Tolerance Specifications in Title Block General tolerance note specifies the tolerance for all unspecified toleranced dimensions. 4. Notes Referring to Specific Conditions : 4. Notes Referring to Specific Conditions General Tolerances could be in the form of a note similar to the one shown below: ALL DECIMAL DIMENSIONS TO BE HELD TO .002" Means that a dimension such as .500 would be assigned a tolerance of 0.002, resulting in a upper limit of .502 and a lower limit of .498 Important Terms – Single Part : Important Terms – Single Part Nominal Size – general size, usually expressed in common fractions (1/2" for the slot) Basic Size – theoretical size used as starting point (.500" for the slot) Actual Size – measured size of the finished part (.501" for the slot) Important Terms – Single Part : Important Terms – Single Part Limits – maximum and minimum sizes shown by tolerances (.502 and .498 – larger value is the upper limit and the smaller value is the lower limit, for the slot) Tolerance – total allowable variance in dimensions (upper limit – lower limit) – object dimension could be as big as the upper limit or as small as the lower limit or anywhere in between Important Terms – Multiple Parts : Important Terms – Multiple Parts Allowance – the minimum clearance or maximum interference between parts Fit – degree of tightness between two parts Clearance Fit – tolerance of mating parts always leave a space Interference Fit – tolerance of mating parts always interfere Transition Fit – sometimes interfere, sometimes clear Fitting Multiple Parts : Fitting Multiple Parts Fitting Multiple Parts : Fitting Multiple Parts Shaft and Hole Fits : Shaft and Hole Fits Clearance Interference Shaft and Hole Fits : Shaft and Hole Fits Transition Fit Between Parts : Fit Between Parts Clearance Fit Interference Fit Transition Fit Clearance fit: The shaft maximum diameter is smaller than the hole minimum diameter. Interference fit: The shaft minimum diameter is larger than the hole maximum diameter. Transition fit: The shaft maximum diameter and hole minimum have an interference fit, while the shaft minimum diameter and hole maximum diameter have a clearance fit Standard Precision Fits: English Units : Standard Precision Fits: English Units Running and sliding fits (RC) Clearance locational fits (LC) Transition locational fits (LT) Interference locational fits (LN) Force and shrink fits (FN) Fits: ISO Tolerance Distribution : Fits: ISO Tolerance Distribution In the ISO system the distribution of the tolerance relative to the basis size is controlled by the addition of a letter to the specification. e.g.  10 H ( means “diameter”) Upper case letters A to Z (with no I but with a Js) are used for holes and lower case letters a to z (with no i but a js) for shafts. In general, the later the letter in the alphabet, the more material is involved (smaller holes and larger shafts). ISO Tolerance Distribution : Shafts : ISO Tolerance Distribution : Shafts ISO Tolerance Distribution: Holes : ISO Tolerance Distribution: Holes ISO Tolerance Distribution (cont.) : ISO Tolerance Distribution (cont.) The limit (or deviation) nearest to the basic size is termed the fundamental deviation and these are tabulated. In the H case the minimum hole diameter is the basic size while in the h case the largest shaft diameter is the basic size. In the J case the maximum hole size corresponds to the basic size while in the j case the minimum shaft size corresponds to the basic size. Thus, the fundamental deviations for these cases (H, h, J and j) are zero. ISO Tolerance Distribution (cont) : ISO Tolerance Distribution (cont) The Js and js designations are used to specify symmetrical bi-lateral tolerances about the basic size. These are often used where no fit is involved. The ISO system may be used directly to specify a diameter with tolerances and fundamental deviation. For example, "  10 H 8 " indicates a hole with 10 mm basic size (diameter , zero fundamental deviation and an IT Number of 8. Fits : Fits The choice of fit reflects the functional relationship between the parts e.g. for a bearing fit too tight (too little clearance) -> high friction and overheating or seizure too loose (too much clearance) -> poor constraint, loss of lubricant & wear Terminology for Fits : Terminology for Fits Fits involve the relationship between two components, characterized as hole and a shaft. Clearance (diametral) - actual difference between shaft and hole diameters when hole is larger than shaft Interference (diametral) - actual difference between shaft and hole diameters, when hole is smaller than shaft Allowance (diametral)-specified minimum clearance or maximum interference (MMC) (used to assess assemblability) Terminology for Fits : Terminology for Fits Clearance Fit- always positive clearance (relative motion or easy assembly) Transition Fit- clearance or interference (generally location or assembly) Interference Fit- always positive interference (needs force to disassemble) Hole or Shaft Basis for Fits : Hole or Shaft Basis for Fits Often necessary to have a variety of fits in given product or facility. Two possibilities: or Hole basis usually preferred because of cost of tools to make holes ISO Fits : ISO Fits To control a fit a basic size plus tolerances for each member must be specified In the ISO system this is done using the basic size plus the tolerance distribution letter and the IT number for the hole (uppercase letter) and for the shaft (lower case letter) separated by a slash. e.g.  10 H7/h6 To simplify the choice of fits, standard fits suitable for many applications are defined Preferred ISO Fits (clearance) : Preferred ISO Fits (clearance) Preferred ISO Fits (clearance) : Preferred ISO Fits (clearance) Preferred ISO Fits (transition) : Preferred ISO Fits (transition) Preferred ISO Fits (transition) : Preferred ISO Fits (transition) Preferred ISO Fits (interference) : Preferred ISO Fits (interference) Need for Surface Finish Control : Need for Surface Finish Control Fatigue Corrosion Tribology (lubrication, friction and wear) Fluid flow Appearance Surface Finish Measurement : Surface Finish Measurement Common measurement devices Stylus Optical Many quantitative measures of finish (50 or more) Most common roughness measure is arithmetic mean value Ra (previously called arithmetic average or center line average) Basic Hole System or Hole Basis : Basic Hole System or Hole Basis Definition of the "Basic Hole System": The "minimum size" of the hole is equal to the "basic size" of the fit Example: If the nominal size of a fit is 1/2", then the minimum size of the hole in the system will be 0.500" Basic Hole System : Basic Hole System Clearance = Hole – Shaft Cmax = H____ – S____ Cmin = H____ – S____ Fill in the subscripts (min, max) in the equations above. Basic Hole System : Basic Hole System Clearance = Hole – Shaft Cmax = Hmax – Smin Cmin = Hmin – Smax Both Cmax and Cmin <0 – _________ fit Both Cmax and Cmin >0 – _________ fit Cmax > 0; Cmin < 0 – ___________ fit What types of fits are these? Basic Hole System : Basic Hole System Clearance = Hole – Shaft Cmax = Hmax – Smin Cmin = Hmin – Smax Both Cmax and Cmin <0 – Interference fit Both Cmax and Cmin >0 – Clearance fit Cmax > 0; Cmin < 0 – Transition fit System Tolerance = Cmax – Cmin Allowance = Min. Clearance = Cmin Basic Hole System : Basic Hole System Calculate Maximum and Minimum Clearance Clearance = Hole – Shaft Cmax = Hmax – Smin Cmax = .510 – .485 = .025 Cmin = .505 – .490 = .015 What type of fit is this? Cmax > Cmin > 0 Clearance Cmin = Hmin – Smax Metric Limits and Fits : Metric Limits and Fits Based on Standard Basic Sizes – ISO Standard, Note that in the Metric system: Nominal Size = Basic Size Example: If the nominal size is 8, then the basic size is 8 Metric Preferred Hole Basis System of Fits : Metric Preferred Hole Basis System of Fits Metric Tolerance Homework – TOL-1B : ???? ???? 7.960 7.924 Metric Tolerance Homework – TOL-1B Free Running H9/d9 Basic Size: 8 (1) Nominal Size: ? (1) Nominal Size: 8 (2) Shaft Limits: (7) Minimum Clearance: ???? (8) Maximum Clearance: ???? (3) Shaft Tolerance: ???? (3) Shaft Tolerance: 0.036 (4) Hole Limits: ???? ???? 8.036 8.000 (5) Hole Tolerance: ???? (5) Hole Tolerance: 0.036 (6) Ts: ???? (6) Ts: 0.072 (7) Minimum Clearance: 0.040 (8) Maximum Clearance: 0.112 CHECK: Ts = Cmax – Cmin? CHECK: 0.072 = 0.112 – 0.040 = 0.072 Slide 43: Fits and Tolerances andMultiple Assembly Fits Objectives : Objectives Review terms for single and multiple parts Review equations for Cmax and Cmin Tolerancing example for more than two parts Tolerancing example for more than two parts with tolerance distribution Review – Definition of Tolerance : Review – Definition of Tolerance Tolerance is the total amount a dimension may vary. It is the difference between the maximum and minimum limits. There is no such thing as an "exact size". Tolerance is key to interchangeable parts. Direct limits and tolerance values : Direct limits and tolerance values Can be: Limits: Upper limit – 3.53 Lower limit – 3.49 Unilateral – vary in only one direction 3.49 0 -.0X +.0X - 0 Bilateral – vary larger or smaller (may or may not be same amount) 3.50 +.05 -.01, +.10 -.20 +/- 0.05 +.04 0 +.03 -.01 Important Terms – Single Part : Important Terms – Single Part Nominal Size – general size (1/2") Basic Size – theoretical size used as starting point (.500") Actual Size – measured (.501") Limits – maximum and minimum sizes shown by tolerances Tolerance – total allowable variance in dimensions (upper – lower limit) Important Terms – Multiple Parts : Important Terms – Multiple Parts Allowance – the minimum clearance or maximum interference between parts Fit – degree of tightness between two parts Clearance Fit – tolerance of mating parts always leave a space Interference Fit – tolerance of mating parts always interfere Transition Fit – sometimes interfere, sometimes clear Tolerance – total allowable variance in dimensions (upper – lower limit) Review – Fitting Two Parts : Review – Fitting Two Parts Review – Shaft and Hole Fits : Review – Shaft and Hole Fits Clearance Interference Review – Shaft and Hole Fits : Review – Shaft and Hole Fits Transition Review – Basic Hole System : Review – Basic Hole System System Tolerance = Cmax - Cmin Sometimes called Clearance Tolerance Also, System Tolerance = Σ Ti So, System Tolerance, or Ts, can be written as: Ts = Cmax - Cmin = Σ Ti Thus, you always have a check value More than Two Parts : ______ - ( ______ + ______ ) More than Two Parts ______ - ( ______ + ______ ) Hmax Hmax Hmin Hmin S1max S1max S1min S1min S2max S2max S2min S2min S1 S2 H C Cmax = Cmin = More than Two Parts – System Tolerance : More than Two Parts – System Tolerance + + TH = Hmax - Hmin TH TS1 = S1max - S1min TS2 = S2max - S2min TS1 TS2 TSYS = Cmax - Cmin = TH + TS1 + TS2 = TSYS ______ - ( ______ + ______ ) ______ - ( ______ + ______ ) Hmax Hmin S1max S1min S2max S2min S1 S2 H C Cmax = Cmin = More than Two Parts : More than Two Parts Hmax Hmin S1 S2 H C Cmax Cmin Cmax = 0.1+0.05 = 0.15 Cmin = 0.1 - 0.05 = 0.05 S1max = 1.50+0.01 = 1.51 S1min = 1.50 - 0.03 = 1.47 S2max = 1.18 S2min = 1.15 1.51 1.47 1.18 1.15 0.15 0.05 Hmax = 0.15 + 1.47 + 1.15 = 2.77 Hmin = 0.05 + 1.51 + 1.18 = 2.74 2.74 2.77 ( S1min + S2min ) ( S1max + S2max ) + + - - = = More than Two Parts : ______ - ( ______ + ______ ) More than Two Parts ______ - ( ______ + ______ ) Hmax Hmin S1max S1min S2max S2min S1 S2 H C Cmax = Cmin = Now Check: TH = Hmax - Hmin = 0.03 TS1 = S1max - S1min = 0.04 TS2 = S2max - S2min = 0.03 TH + TS1 + TS2 = 0.10 Cmax – Cmin = 0.10 1.51 1.47 1.18 1.15 0.15 = 0.05 = ______ - ( ______ + ______ ) ______ - ( ______ + ______ ) 2.74 2.77 + + 0.03 0.04 0.03 = Cmax – Cmin = 0.10 Tolerance Distribution : _____ = ______ - ( ______ + ______ ) _____ = ______ - ( ______ + ______ ) Tolerance Distribution S1 S2 H C Cmax = 0.15 + 0.05 = 0.20 Cmin = 0.15 – 0.05 = 0.10 0.20 0.10 We have 0.10 tolerance to distribute to H, S1, and S2 Given: C = 0.15 ± 0.05 S1 = 2.50 (nom) H = 3.65 (nom) Find: Hmin, Hmax, S1min, S1max, S2min, S2max Tolerance Distribution : _____ = ______ - ( ______ + ______ ) _____ = ______ - ( ______ + ______ ) Tolerance Distribution S1 S2 H C 0.20 0.10 Let’s make TH = 0.04 = + 0.04 Let’s make Hmin = 3.65 3.65 So: Hmax = Hmin + TH = 3.65 + 0.04 = 3.69 3.69 Given: C = 0.15 ± 0.05 S1 = 2.50 (nom) H = 3.65 (nom) Find: Hmin, Hmax, S1min, S1max, S2min, S2max Tolerance Distribution : _____ = ______ - ( ______ + ______ ) _____ = ______ - ( ______ + ______ ) Tolerance Distribution S1 S2 H C 0.20 0.10 Let’s make TS1 = 0.03 = + + 0.04 Let’s make S1max = 2.51 3.65 So: S1min = S1max - TH = 2.51 - 0.03 = 2.48 3.69 0.03 2.51 2.48 Given: C = 0.15 ± 0.05 S1 = 2.50 (nom) H = 3.65 (nom) Find: Hmin, Hmax, S1min, S1max, S2min, S2max Tolerance Distribution : _____ = ______ - ( ______ + ______ ) _____ = ______ - ( ______ + ______ ) Given: C = 0.15 ± 0.05 S1 = 2.50 (nom) H = 3.65 (nom) Find: Hmin, Hmax, S1min, S1max, S2min, S2max Tolerance Distribution S1 S2 H C 0.20 0.10 S2min = Hmax – S1min – Cmax = 3.69 – 2.48 – 0.20 = 1.01 = + + 0.04 3.65 3.69 0.03 2.51 2.48 S2max = Hmin – S1max – Cmin = 3.65 – 2.51 – 0.10 = 1.04 1.01 1.04 Check: TS2 = S2max – S2min = 1.04 – 1.01 = 0.03 0.03

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